Is it a crazy idea to cut out the holding tank and install a deck plate in order to clean (shop vac) the bottom of the tank?
Nope, not a crazy idea, but there's a a much easier way to flush out a waste tank without one: With the tank empty, add enough water to the tank via the pumpout fitting--'cuz that sends the water into the tank at the bottom to stir up any sludge--to cover the bottom to a depth
of 4-5 inches...pump that out. Repeat...repeat...till you're pumping out clean water. Do it one more time and this time turn on any macerator pump
to rinse it and any associated plumbing
This should be done 2-3x season and especially in preparation for winter or other extended layup
What's the black tank made of?
On a Catalina
both your water and waste tanks
are most likely the same rotomolded PE tank, made by Ronco Plastics 'cuz they've been Catalina's water and waste tank supplier for decades. Catalina just specs black as their holding tank
color. (An yes, it's vented...USCG regs require that all waste tanks
be vented to the outside of the boat
because methane is flammable).
Any special adhesive sealant to be used on the black tank?
Nope...just the same rubber gasket
that should have been supplied with the inspection ports
you put on your water tank. Beckson has inspection ports
with gaskets and threaded caps with rubber o-rings.